Respiratory syncytial virus (RSV) infection remains an important medical problem for infants and the elderly. Infection requires the fusion of the vira membrane with the target cell membrane, which delivers the viral genome into the target cell cytoplasm. The viral fusion (F) protein, one of the three RSV glycoproteins, performs this role. It is clear from the crystal structures of two related viruses, one in the pre-triggered and one in th post-triggered form, that the F protein undergoes massive changes in structure to accomplish membrane fusion. Based on these structures, models of the pre-triggered and post-triggered RSV F protein have been generated. Using the pre-triggered model, the most likely sites for attachment to target cells and/or for triggering have been identified. Mutation of the amino acids in these sites to alanine will test their importance in cell-cell fusion. Mutations that prevent fuion without preventing the F protein from reaching the cell surface will be studied biochemically for their attachment and triggering abilities. For these studies, a soluble (sF) version of the F proten in its pre-triggered form has been produced for the first time, and shown to be triggered by a reduction in molarity. The possibility that reduced molarity causes F protein triggering will also be investigated since its mechanism may provide novel antiviral drug targets. Overall, these studies will result in the identification of functional domains in the F protein that can be used t rapidly screen for novel antiviral compounds and to develop novel vaccines.